1. Field of the Invention
The invention relates to liquid crystal displays (LCDs). More specifically, the invention describes a method and apparatus for detecting flicker in a digital image displayed on a liquid crystal display.
2. Discussion of Related Art
Liquid crystal displays (LCDs) are significantly lighter in weight and slimmer, consume far less energy and can reproduce a wider range of colors than any competing technologies. Accordingly, LCDs are increasingly being used for the display device in televisions, personal computers, etc., and in many state-of-the-art equipment such as automotive navigation systems and simulation devices.
Using contemporary LCD technology, an electric field is applied to liquid crystal material having an anisotropic dielectricity that is injected between two substrates (an array substrate and a counter substrate) that are arranged substantially parallel to one another with a predetermined gap between them. A displayed image is obtained by controlling an intensity of the electric field that, in turn, controls the amount of light permeating the substrates. In contrast to passive matrix type LCDs, active matrix type LCDs include a plurality of gate lines placed parallel to one another disposed on a substrate and a plurality of data lines insulated from and crossing the gate lines. A number of pixel electrodes are formed corresponding to respective regions defined by the intersecting data lines and gate lines. Furthermore, a thin film transistor (TFT) is provided near each of the intersections of the gate lines and the data lines. Each pixel electrode is connected to a data line via a corresponding TFT, the TFT serving as a switching device. Typically, each TFT has a gate electrode, a drain electrode, and a source electrode where the pixel electrodes are connected to the drain electrodes. The electric field applied to the liquid crystal material is generated by a difference in levels of a common voltage and a data voltage applied respectively to the common electrodes and the pixel electrodes in the LCD such that the intensity of the electric field is controlled by changing data voltage or common voltage levels.
Since the liquid crystal material degrades if the electric field is applied to the liquid crystal material continuously in the same direction, the direction in which the electric field is applied must be constantly changed. Namely, a value of the data voltage minus the common voltage must be repeatedly alternated from a positive value (hereinafter referred to as positive voltage) to a negative value (hereinafter referred to as negative voltage). Such a switching of electrode voltage values between positive and negative values is referred to as inversion drive. Among the different types of inversion drive methods are frame inversion, line inversion, dot inversion, and column inversion methods. In frame inversion, for example, (in which the polarity of data voltage is inverted to frame cycles (typically 60 Hz), positive voltage is applied in odd frames, while negative voltage is applied in even frames.
Unfortunately, however, what is referred to as a kickback voltage is generated by parasitic capacitance in the pixels such that the RMS of the positive voltage is different from the RMS of the negative voltage. Accordingly, the amount of light permeating the liquid crystal material in the odd frames and that of light permeating the liquid crystal material in the even frames is different resulting in what is commonly referred to as screen (or luminance) flicker observed in units of one-half of frame frequency of, for instance, 60 Hz (or 30 Hz).
LCD, or luminance, flicker (which is inherent in the majority of LCD flat panels when) has been a primary concern for applications that require the display of high contrast, high density, moving data in that the continual luminance flicker can cause serious eye fatigue to the user resulting in difficulty in interpreting the displayed information, for example. Since flicker is inherent in the majority of LCD flat panels but varies with a number of factors, such a refresh rate, displayed motion, etc. various systems for identifying particular frames of video data having a high likelihood of a displayed image having an unacceptable amount of flicker have been developed. One such system 100 is illustrated in FIG. 1A showing a conventional approach to detecting flicker in an image to be displayed on an LCD flat panel screen 102. As shown in FIG. 1A, in an attempt to identify a xe2x80x9cbadxe2x80x9d flicker pattern formed of a number of xe2x80x9cbadxe2x80x9d pixel pairs, the flat panel screen 102 (which for this example is 1024 pixels by 768 pixels) is divided into a number of blocks 104 which are, in turn, further divided into segments 106. In this example, each of the segments 106 is 64 pixels wide for a total of 16 segments per frameline (of which there are 768) for a total of 12288 segments.
Using the system 100, each of the segments 106 are tested for a number X of xe2x80x9cbadxe2x80x9d pixel pairs included therein. The number of bad pixel pairs per segment is then compared to a pre-determined bad segment threshold number Xt which determines whether or not a particular segment is classified as a xe2x80x9cbad segmentxe2x80x9d. Once the number and location of bad segments within each block is determined, an evaluation is made on a block by block basis of the number of bad segments per block. The result of this evaluation is compiled into what is referred to as a bad segment number which, in turn, is used to ultimately identify bad frames, or those frames prone to produce flicker on the flat panel screen 102.
This situation is best illustrated in FIG. 1B, showing the flat panel screen 102 having a number of segments 106 identified as bad segments 108. Although the system 100 is capable of identifying potential a flicker inducing pattern 110 such as that shown to be within the block 104-1 where the bad pixel pairs conveniently fall within a predefined segment, the system 100, however, can not identify a pattern 112 where associated bad pixel pairs are included in more than one segment and/or cross block boundaries.
Therefore what is desired is an efficient method and apparatus for identifying flicker prone patterns in an image to be displayed on an LCD monitor.
According to the present invention, methods, apparatus, and systems are disclosed for identifying flicker prone patterns in an image to be displayed on an LCD monitor are disclosed.
In one embodiment, a flicker pattern detector coupled to a video signal source suitable for detecting a sub-pixel pair susceptible to producing a flicker event in an image displayed on a liquid crystal display (LCD) unit is described. The flicker pattern detector includes a two dimensional flicker pattern analyzer arranged to perform a two dimensional flicker pattern analysis on a selected group of sub-pixels some of which are included in a first plurality of sub-pixels that includes a first current sub-pixel and a first next sub-pixel included in a first video frameline and a remainder of which are included in a second plurality of sub-pixels included in a second video frameline that is received, in real time, from the video signal source that includes a second current sub-pixel and a second next sub-pixel. The two dimensional flicker pattern analyzer includes a first storage device suitable for storing the first plurality of sub-pixels, a second storage device coupled to the first storage device suitably arranged to store the first current sub-pixel, a third storage device arranged to store a the second current sub-pixel, and a comparator unit coupled to the first storage device, the second storage device and the third storage device. The comparator unit is arranged to perform a two dimensional compare operation, and update a final flicker frame score based upon the compare operation indicative of the susceptibility of producing a flicker event in an image displayed on a liquid crystal display (LCD) unit.
In a preferred embodiment, the flicker detector also includes a one dimensional flicker pattern analyzer arranged to perform a one dimensional flicker pattern analysis on a previous sub-pixel and a current sub-pixel that includes a fourth storage device suitable for storing the previous sub-pixel, a second comparator unit coupled to the fourth storage device arranged to compare the previous sub-pixel and a current sub-pixel received in real time from the video signal source and based upon the compare, updates the final flicker frame score.
In another embodiment, a method for detecting a sub-pixel pair susceptible of producing a flicker event in an image from a video signal source displayed on a liquid crystal display (LCD) unit. A two dimensional flicker pattern analysis is performed on a selected group of sub-pixels some of which are included in a first plurality of sub-pixels that includes a first current sub-pixel and a first next sub-pixel included in a first video frameline and a remainder of which are included in a second plurality of sub-pixels included in a second video frameline that is received, in real time, from the video signal source that includes a second current sub-pixel and a second current sub-pixel.
In a preferred embodiment, the first current sub-pixel is compared to the first previous sub-pixel, the first current sub-pixel is compared to the second current sub-pixel, the second current sub-pixel is compared to the second previous sub-pixel, and the second previous sub-pixel is compared to the first previous sub-pixel. The flicker frame score is updated based upon the comparisons.
In yet another embodiment, computer program product for enabling a computer to perform a method for detecting a sub-pixel pair susceptible of producing a flicker event in an image from a video signal source displayed on a liquid crystal display (LCD) unit is disclosed. The computer program product includes computer code for performing a two dimensional flicker pattern analysis on a selected group of sub-pixels some of which are included in a first plurality of sub-pixels that includes a first current sub-pixel and a first previous sub-pixel included in a first video frameline and a remainder of which are included in a second plurality of sub-pixels included in a second video frameline that is received, in real time, from the video signal source that includes a second current sub-pixel and a second previous sub-pixel and computer readable medium for storing the computer code.